The invention relates to an operational amplifier comprising at least a first amplifier stage, a second amplifier stage, which is driven by the first amplifier stage, and a first and a second junction point between which a capacitive signal path, for improving the high-frequency properties of the operational amplifier, is included or is to be included.
When two amplifier stages are connected one after the other in an operational amplifier--the first amplifier stage generally having the larger bandwith--the problem arises that the combined gain of the two stages exhibits a roll-off of 12 dB per octave over a comparatively wide range and especially in the frequency range where the open-loop gain of the combination of the two stages decreases to unity, so that the provision of feedback across the operational amplifier may give rise to instabilities. In order to preclude this problem, high-frequency correction networks are included in operational amplifiers. This is frequently left at the discretion of the user of said operational amplifier by the provision of suitable terminals.
A first known correction method is inter alia eemployed in an integrated circuit which is commercially available under type number .mu.A 709 and which is described in "Philips Data Handbook-Signetics integrated circuits 1978", pages 100-106. In accordance with this method, the bandwidth of the first amplifier stage is made smaller than the bandwidth of the second stage by means of an RC-network in such a way that the gain of the first stage decreases to unity. The high-frequency roll-off is then determined by the second stage and is then substantially 6 dB per octave. In the case of a suitable choice of said RC-network, the roll-off in gain over the entire frequency range beyond the frequency at which the gain of the combination has decreased to unity is substantially equal to 6 dB per octave.
A similar effect can be obtained by another correction method, which is inter alia employed in an integrated circuit which is available under type number .mu.A 741 and which is described in the Handbook on page 60-65. Here use is made of the Miller effect, enabling a capacitor with a small capacitance to be used, which facilitates integration of said capacitor. The second amplifier stage is then shunted by the capacitor. Said second amplifier stage is then inverting--which is essential for the Miller effect--and the impedance of the output of the second stage, seen from said capacitor, is comparatively low relative to the impedance of the output of the first stage, again seen from the capacitor. The effect of this step may be described as a reduction of the bandwidth of the first stage, because, for high frequencies the output of said first stage is short-circuited to the comparatively low-ohmic output of the second stage via said capacitor, and the integrating effect of the second stage to which negative feedback is applied via said capacitor. The result of said step corresponds to the result of the first-mentioned step.
Drawbacks of these known and frequently employed correction methods are that the bandwidth of the combination is limited to the bandwidth of said second stage, which, when these correction methods are employed, also has the smaller bandwidth of the two amplifier stages, and that the signal-to-noise ratio of the output signal at higher frequencies deteriorates towards the input of the first amplifier stage, because the reduction of the bandwidth of the first stage does influence the signal but not the noise contribution of the second stage. Here, bandwidth is to be understood to mean the bandwidth up to which the gain has decreased to unity.
These drawbacks are recognized in an article by T.J. van Kessel in "IEEE Journal of Solid-State Circuits", Vol. SC-3, No. 4, December 1968, pages 348-352 entitled "An integrated operational amplifier with novel HF behaviour". This article proposes an adequate reduction of the bandwidth of the second stage relative to the bandwidth of the first stage by means of a capacitance and an addition of the output signal of the first stage to the output signal of the second stage via a parallel signal path, in such a way that for frequencies where the gain of said second stage has rolled off substantially, the gain of the combination is equal to the gain of the first stage, so that the bandwidth of the combination is equal to the bandwidth of the first stage, which has the larger bandwidth of the two stages. In the case of a suitable choice of the frequency roll-off of the second stage, this also yields a roll-off of 6 dB per octave over the entire frequency range up to the frequency where the gain of the combination has decreased to unity. In this case, in contradistinction to the first-mentioned method, the bandwidth at which the gain of the combination has decreased to unity is equal to the bandwidth of the wide-band first stage and this correction method does not have the influence on the signal-to noise ratio.
In said article this correction method, which has been described in general terms, is illustrated on the basis of a specific operational amplifier, which has the drawback that said bypass path is comparatively high-ohmic because it comprises a resistor which also serves to add the output signals of the first and the second stage to each other. Especially the high frequency signal component becomes therefore available across a comparatively high impedance, which in its turn has the drawback that it is difficult to feed the output signal to an output of the operational amplifier via a buffer stage, specifically an emitter-follower or a class-B output stage, because then the input capacitance of said buffer stage together with the value of said resistor will give rise to an excessive time constant.